3,4,5-trifluoropheny 4-cyclohexylbenzoates

ABSTRACT

3,4,5-trifluorophenyl 4-cyclobenzoates of formula I    &lt;IMAGE&gt;  I  in which R1 is an alkyl or alkenyl radical having up to 18 carbon atoms which may be substituted, and L1 is H or F. Liquid-crystalline media comprising such 3,4,5-trifluorophenyl 4-cyclohexylbenzoates, and electro-optical displays containing a liquid-crystalline medium of this type.

BACKGROUND OF THE INVENTION

The present invention relates to 3,4,5-trifluorophenyl4-cyclohexylbenzoates, to a liquid-crystalline medium, to the usethereof for electro-optical purposes, and to displays containing thismedium.

Liquid crystals are used, in particular, as dielectrics in displaydevices, since the optical properties of such substances can be modifiedby an applied voltage. Electro-optical devices based on liquid crystalsare extremely well known to the person skilled in the art and can bebased on various effects. Examples of such devices are cells havingdynamic scattering, DAP (deformation of aligned phases) cells,guest/host cells, TN (twisted nematic) cells, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The most common display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should haverelatively low viscosity and give short addressing times, low thresholdvoltages and high contrast in the cells.

Furthermore, they should have a suitable mesophase, for example anematic or cholesteric mesophase for the abovementioned cells, at normaloperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used in theform of mixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, must satisfy different requirements depending on thecell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, media of large positive dielectric anisotropy, broadnematic phases, relatively low birefringence, very high resistivity,good UV and temperature stability and low vapour pressure are desiredfor matrix liquid-crystal displays having integrated non-linear elementsfor switching individual pixels (MLC displays).

Matrix liquid-crystal displays of this type are known. Examples ofnonlinear elements which can be used for individual switching of singlepixels are active elements (i.e. transistors). This is then referred toas an "active matrix", and a differentiation can be made between twotypes:

1. MOS (metal oxide semiconductor) or other diodes on silicon wafers assubstrate.

2. Thin-film transistors (TFTs) on a glass plate as substrate.

Use of monocrystalline silicon as the substrate material limits thedisplay-size, since even modular assembly of the various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A differentiationis made between two technologies: TFTs comprising compoundsemiconductors, such as, for example, CdSe, or TFTs based onpoly-crystalline or amorphous silicon. Intensive work is being carriedout worldwide on the latter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, whilst the other glass plate carries the transparentcounterelectrode on the inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fullycolour-compatible image displays, in which a mosaic of red, green andblue filters is arranged in such a way that each filter element islocated opposite a switchable pixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display containingintegrated nonlinear elements, i.e., in addition to the active matrix,also displays containing passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TV sets) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. In addition to problems with respect to the angledependence of the contrast and the response times, problems arise in MLCdisplays owing to inadequate resistivity of the liquid-crystal mixturesTOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, Sept. 1984:A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff,Paris; STROMER, M., Proc. Eurodisplay 84, Sept. 1984: Design of ThinFilm Transistors for Matrix Addressing of Television Liquid CrystalDisplays, p. 145 ff, Paris!. With decreasing resistance, the contrast ofan MLC display drops, and the problem of after-image elimination canoccur. Since the resistivity of the liquid-crystal mixture generallydrops over the life of an MLC display owing to interaction with theinternal surfaces of the display, a high (initial) resistance is veryimportant in order to obtain acceptable service lives. In particular inthe case of low-voltage mixtures, it was hitherto not possible toachieve very high resistivities. It is furthermore important that theresistivity increases as little as possible with increasing temperatureand after heating and/or exposure to radiation. Also particularlydisadvantageous are the low-temperature properties of the mixtures fromthe prior art. It is required that crystallization and/or smectic phasesdo not occur, even at low temperatures, and that the temperaturedependence of the viscosity is as low as possible. The MLC displays ofthe prior art thus do not satisfy current requirements.

There thus continues to be a great demand for MLC displays having veryhigh resistivity at the same time as a broad operating temperaturerange, short response times, even at low temperatures, and low thresholdvoltage which do not have these disadvantages or only do so to a reducedextent.

In the case of TN (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

broadened nematic phase range (in particular down to low temperatures),

switchability at extremely low temperatures (outdoor use, automobiles,avionics),

increased stability on exposure to UV radiation (longer life).

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted cells (STN), media are desired which enablegreater multiplexibility and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther extension of the parameter latitude available (clearing point,smectic-nematic transition or melting point, viscosity, dielectricquantities, elastic quantities) is urgently desired.

SUMMARY OF THE INVENTION

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the abovementioneddisadvantages, or only do so to a reduced extent, and preferably at thesame time have very high resistivities and low threshold voltages.

WO 92/05 230 discloses compounds of the formula ##STR2## but these havecomparatively low dielectric anisotropy.

It has now been found that this object can be achieved when novel mediaare used in displays.

The invention thus relates to 3,4,5-tri-fluorophenyl4-cyclohexylbenzoates of the formula I, ##STR3## in which

R¹ is an alkyl or alkenyl radical having up to 18 carbon atoms which isunsubstituted, monosubstituted by CN or CF₃ or at least monosubstitutedby halogen, it also being possible for one or more CH₂ groups in theseradicals to be replaced by --O--, --S--, --C.tbd.C-- --OC--O-- or--O--CO-- in such a way that O atoms are not linked directly to oneanother, and

L¹ is H or F.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, these compounds can serve asbase materials from which liquid-crystalline media are predominantlycomposed; however, compounds of the formula I can also be added toliquid-crystalline base materials from other classes of compound inorder, for example, to modify the dielectric and/or optical anisotropyof a dielectric of this type and/or to optimize its threshold voltageand/or its viscosity.

In the pure state, the compounds of the formula I are colorless and formliquid-crystalline mesophases in a temperature range which is favourablylocated for electro-optical use. They are stable chemically, thermallyand to light.

The invention relates in particular to the compounds of the formula I inwhich R¹ is alkyl having 1 to 10 carbon atoms.

Particular preference is given to compounds of the formula I in which L¹is F.

If R is an alkyl radical and/or an alkoxy radical, this can bestraight-chain or branched. It is preferably straight-chain, has 2, 3,4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl,butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxyor heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl,tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy,undecoxy, dodecoxy, tridecoxy or tetradecoxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R is an alkenyl radical, this can be straight-chain or branched. Itis preferably straight-chain and has 2 to 10 carbon atoms. Accordingly,it is in particular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl,pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-,-2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6-or-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-,-2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

If R is an alkyl radical in which one CH₂ group has been replaced by--O-- and one has been replaced by --CO--, these are preferablyadjacent. These thus contain one acyloxy group --CO--O-- or oneoxycarbonyl group --O--CO--. These are preferably straight-chain andhave 2 to 6 carbon atoms.

Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy,pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl,utyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl,2-propionyloxy-ethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionylorypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonyl-methyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R is an alkyl or alkenyl radical which is monosubstituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CNor CF₃ is in any desired position.

If R is an alkyl or alkenyl radical which is at least monosubstituted byhalogen, this radical is preferably straight-chain and halogen ispreferably F or Cl. In the case of multiple substitution, halogen ispreferably F. The resultant radicals also include perfluorinatedradicals. In the case of monosubstitution, the fluorine or chlorinesubstituent can be in any desired position, but preferably in theω-position.

Compounds of the formula I containing branched wing groups R mayoccasionally be of importance due to better solubility in the customaryliquid-crystalline base materials, but in particular as chiral dopes ifthey are optically active. Smectic compounds of this type are suitableas components of ferroelectric materials.

Compounds of the formula I having S_(A) phases are suitable, forexample, for thermally addressed displays.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2 -methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy,1-methylhexoxy and 1-methylheptoxy.

Preferred smaller groups of compounds of the formula I are those of thesubformulae I1 and I2 L¹ =H or F!: ##STR4##

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag,Stuttgart), to be precise under reaction conditions which are known andsuitable for said reactions. Use can also be made here of variants whichare known per but which are not mentioned here in greater detail.

The novel compounds can be prepared, for example, as shown in thefollowing scheme: ##STR5##

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates which,together with a frame, form a cell, integrated nonlinear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highresistivity located in the cell) which contain media of this type, andto the use of these media for electro-optical purposes.

The novel liquid-crystal mixtures facilitate a significant broadening ofthe parameter latitude available.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and dielectric anisotropy are farsuperior to previous materials from the prior art.

The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δε was previously only achievable to anunsatisfactory extent. Although systems such as, for example, ZLI-3119have a comparable clearing point and comparatively favourableviscosities, they have, however, a Δε of only +3.

Other mixture systems have comparable viscosities and values of Δε, butonly have clearing points in the region of 60° C.

The liquid-crystal mixtures according to the invention make it possibleto achieve clearing points of above 80°, preferably above 90°,particularly preferably above 100° C., and simultaneously dielectricanisotropy values Δε≧6, preferably ≧8, and a high value for theresistivity while retaining the nematic phase down to -20° C. andpreferably down to -30° C., particularly preferably down to -40° C.,which allows excellent STN and MLC displays to be achieved. Inparticular, the mixtures are characterized by low operating voltages.The TN thresholds are below 2.0 V, preferably below 1.5 V, particularlypreferably <1.3 V.

It goes without saying that a suitable choice of the components of themixtures according to the invention also allows higher clearing points(for example above 110°) to be achieved at higher threshold voltages orlower clearing points to be achieved at lower threshold voltages whileretaining the other advantageous properties. It is likewise possible toobtain mixtures of relatively high Δε and thus relatively low thresholdsif the viscosities are increased by a correspondingly small amount. Thenovel MLC displays preferably operate at the first transmission minimumof Gooch and Tarry C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4,1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584,1975!; in this case, a lower dielectric anisotropy is sufficient inaddition to particularly favourable electro-optical properties, such as,for example, steep gradient of the characteristic line and low angledependency of the contrast (German Patent 30 22 818) at the samethreshold voltage as in an analogous display at the second minimum. Thisallows significantly higher resistivities to be achieved at the firstminimum using the novel mixtures than using mixtures containing cyanocompounds. The person skilled in the art can use simple routine methodsto produce the birefringence necessary for a prespecified layerthickness of the MLC display by a suitable choice of the individualcomponents and their proportions by weight.

The viscosity at 20° C. is preferably <60 mPa.s, particularly preferably<50 mPa.s. The nematic phase range is preferably at least 90°, inparticular at least 100°. This range preferably extends at least from-20° to +80°.

Measurements of the "capacity holding ratio" (HR) S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1989); G. Weber et al., LiquidCrystals 5, 1381 (1989)! have shown that novel mixtures comprisingcompounds of the formula I exhibit a considerably smaller decrease inthe HR with increasing temperature than do analogous mixtures in whichthe compounds of the formula I have been replaced bycyanophenylcyclohexanes of the formula ##STR6## or esters of the formula##STR7##

The UV stability of the novel mixtures is also considerably better, i.e.they exhibit a significantly smaller decrease in the HR on exposure toUV radiation.

The novel media are preferably based on a plurality (preferably two ormore) of compounds of the formula I, i.e. the proportion of thesecompounds is 5-95%, preferably 10-60% and particularly preferably in therange 20-50%.

The individual compounds of the formulae I to XIII and theirsub-formulae which can be used in the novel media are either known orcan be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

Medium comprises compounds of the formula I in which R is preferablyethyl, furthermore propyl, butyl or pentyl. Compounds of the formula Ihaving short side chains R have a positive effect on the elasticconstants, in particular K₁, and give mixtures having particularly lowthreshold voltages

Medium additionally comprises one or more compounds selected from thegroup consisting of the general formulae II to VII: ##STR8## in whichthe individual radicals have the following meanings:

R⁰ : n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having upto 9 carbon atoms,

X⁰ F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6 carbonatoms,

Y¹ and Y² : in each case, independently of one another, H or F,

r: 0 or 1.

The compound of the formula IV is preferably ##STR9##

Medium additionally comprises one or more compounds of the formula##STR10##

Medium additionally comprises one or more compounds selected from thegroup consisting of the general formulae VIII to XIII: ##STR11## inwhich R⁰, X⁰, Y¹ and Y² are each, independently of one another, asdefined above, preferably F, Cl, CF₃, OCF₃, OCHF₂, alkyl, oxaalkyl,fluoro-alkyl or alkenyl, in each case having up to 6 carbon atoms.

The proportion of compounds of the formulae I to VII together is atleast 50% by weight in the total mixture;

The proportion of compounds of the formula I is from 10 to 50% by weightin the total mixture;

The proportion of compounds of the formulae II to VII is from 30 to 70%by weight in the total mixture; ##STR12## is preferably ##STR13##

The medium comprises compounds of the formula II, III, IV, V, VI or VII

R⁰ is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms

The medium essentially consists of compounds of the formulae I to VII

The medium comprises further compounds, preferably selected from thefollowing group consisting of the general formulae XIV to XVII:##STR14## in which R⁰ and X⁰ are as defined above, and the 1,4-phenylenerings may be substituted by CN, chlorine or fluorine. The 1,4-phenylenerings are preferably monosubstituted or polysubstituted by fluorineatoms.

The I: (II+III+IV+V+VI+VII) weight ratio is preferably from 1:10 to 10:1

Medium essentially consists of compounds selected from the groupconsisting of the general formulae I to XIII.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formula II-XVII, resultsin a significant reduction in the threshold voltage and in lowbirefringence values, and at the same time broad nematic phases with lowsmectic-nematic transition temperatures are observed, thus improving theshelf life. The compounds of the formulae I to VII are colourless,stable and readily miscible with one another and with otherliquid-crystal materials.

The term "alkyl" covers straight-chain and branched alkyl groups having1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl,propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atomsare generally preferred.

The term "alkenyl" covers straight-chain and branched alkenyl groupshaving 2-7 carbon atoms, in particular the straight-chain groups.Particularly preferred alkenyl groups are C₂ -C₇ -1E-alkenyl, C₄ -C₇-3E-alkenyl, C₅ -C₇ -4-alkenyl, C₆ -C₇ -5-alkenyl and C₇ -6-alkenyl, inparticular C₂ -C₇ -1E-alkenyl, C₄ -C₇ -3E-alkenyl and C₅ -C₇ -4-alkenyl.Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term "fluoroalkyl" preferably covers straight-chain groupscontaining terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluoro-butyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term "oxaalkyl" preferably covers straight-chain radicals of theformula C_(n) H_(2n+1) --O--(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably 1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the gradient of the transmissioncharacteristic lines, etc., can be modified as desired. For example,1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and thelike generally give shorter addressing times, improved nematictendencies and a higher ratio between the elastic constants k₃₃ (bend)and k₁₁ (splay) compared with alkyl and alkoxy radicals. 4-Alkenylradicals, 3-alkenyl radicals and the like generally give lower thresholdvoltages and lower values of k₃₃ /k₁₁ compared with alkyl and alkoxyradicals.

A --CH₂ CH₂ -- group generally results in higher values of k₃₃ /k₁₁compared with a simple covalent bond. Higher values of k₃₃ /k₁₁facilitate, for example, flatter transmission characteristic lines in TNcells with a 90° twist (for achieving grey tones) and steepertransmission characteristic lines in STN, SBE and OMI cells (greatermultiplexibility), and vice versa.

The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI+VII depends substantially on the desired properties, onthe choice of the components of the formulae I, II, III, IV, V, VIand/or VII and on the choice of any other components which may bepresent. Suitable mixing ratios within the abovementioned range caneasily be determined from case to case.

The total amount of compounds of the formulae I to XIII in the novelmixtures is not crucial. The mixtures may therefore comprise one or morefurther components in order to optimize various properties. However, theeffect observed on the addressing times and the threshold voltage isgenerally greater the higher the total concentration of compounds of theformulae I to XIII.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VII (preferably IIand/or III) in which X⁰ is OCF₃, OCHF₂, F, OCH═CF₂, OCF═CF₂ or OCF₂--CF₂ H. A favorable synergistic effect with the compounds of theformula I results in particularly advantageous properties.

The construction of the novel MLC display from polarizers, electrodebase plates and electrodes with surface treatment corresponds to theconventional construction for displays of this type. The termconventional construction here is broadly drawn and also covers allderivatives and modifications of the MLC display, in particular alsomatrix display elements based on poly-Si TFTs or MIMs.

An essential difference between the novel displays and those customaryhitherto based on the twisted nematic cell is, however, the choice ofthe liquid-crystal parameters in the liquid-crystal layer.

The liquid-crystal mixtures which can be used according to the inventionare prepared in a manner which is conventional per se. In general, thedesired amount of the components used in .the lesser amount is dissolvedin the components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and, after thorough mixing, to remove the solvent again, forexample by distillation.

The dielectrics may also contain other additives known to those skilledin the art and described in the literature. For example, 0-15% ofpleochroic dyes or chiral dopes can be added.

C denotes a crystalline phase, S a smectic phase, S_(C) a smectic Cphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.5 times the value of V₁₀. Δn denotes the optical anisotropy and n_(o)the refractive index. Δε denotes the dielectric anisotropy (Δε=ε.sub.∥-ε.sub.⊥, where ε.sub.∥ is the dielectric constant parallel to thelongitudinal molecular axes and ε.sub.⊥ is the dielectric constantperpendicular thereto). The electro-optical data were measured in a TNcell at the 1st minimum (i.e. at a d·Δn value of 0.5) at 20° C., unlessexpressly stated otherwise. The optical data were measured at 20° C.,unless expressly stated otherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by acronyms, with thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n) H_(2n+1) and C_(m) H_(2m+1) arestraight-chain alkyl radicals containing n or m carbon atomsrespectively. The coding in Table B is self-evident. In Table A, onlythe acronym for the base structure is given. In individual cases, theacronym for the base structure is followed, separated by a hyphen, by acode for the substituents R¹, R², L¹ and L² :

    ______________________________________                                        Code for                                                                      R.sup.1, R.sup.2, L.sup.1, L.sup.2                                                     R.sup.1          R.sup.2    L.sup.1                                                                           L.sup.2                              ______________________________________                                        nm       C.sub.n H.sub.2n+1                                                                             C.sub.m H.sub.2m+1                                                                       H   H                                    nOm      C.sub.n H.sub.2n+1                                                                             OC.sub.m H.sub.2m+1                                                                      H   H                                    nO.m     OC.sub.n H.sub.2n+1                                                                            C.sub.m H.sub.2m+1                                                                       H   H                                    n        C.sub.n H.sub.2n+1                                                                             CN         H   H                                    nN.F     C.sub.n H.sub.2n+1                                                                             CN         H   F                                    nF       C.sub.n H.sub.2n+1                                                                             F          H   H                                    nOF      OC.sub.n H.sub.2n+1                                                                            F          H   H                                    nCl      C.sub.n H.sub.2n+1                                                                             Cl         H   H                                    nF.F     C.sub.n H.sub.2n+1                                                                             F          H   F                                    nF.F.F   C.sub.n H.sub.2n+1                                                                             F          F   F                                    nCF.sub.3                                                                              C.sub.n H.sub.2n+1                                                                             CF.sub.3   H   H                                    nOCF.sub.3                                                                             C.sub.n H.sub.2n+1                                                                             OCF.sub.3  H   H                                    nOCF.sub.2                                                                             C.sub.n H.sub.2n+1                                                                             OCHF.sub.2 H   H                                    nS       C.sub.n H.sub.2n+1                                                                             NCS        H   H                                    rVsN     C.sub.r H.sub.2r+1 --CH═CH--C.sub.5 H.sub.2s --                                            CN         H   H                                    rEsN     C.sub.r H.sub.2r+1 --O--C.sub.2 H.sub.2s --                                                    CN         H   H                                    nAm      C.sub.n H.sub.2n+1                                                                             COOC.sub.m H.sub.2m+1                                                                    H   H                                    nOCCF.sub.2.F.F                                                                        C.sub.n H.sub.2n+1                                                                             OCH.sub.2 CF.sub.2 H                                                                     F   F                                    ______________________________________                                    

Preferred mixture components are shown in Tables A and B.

                  TABLE A                                                         ______________________________________                                         ##STR15##                                                                     ##STR16##                                                                     ##STR17##                                                                     ##STR18##                                                                     ##STR19##                                                                     ##STR20##                                                                     ##STR21##                                                                     ##STR22##                                                                     ##STR23##                                                                     ##STR24##                                                                     ##STR25##                                                                     ##STR26##                                                                     ##STR27##                                                                     ##STR28##                                                                     ##STR29##                                                                     ##STR30##                                                                     ##STR31##                                                                     ##STR32##                                                                     ##STR33##                                                                     ##STR34##                                                                     ##STR35##                                                                    ______________________________________                                    

                  TABLE B                                                         ______________________________________                                         ##STR36##                                                                     ##STR37##                                                                     ##STR38##                                                                     ##STR39##                                                                     ##STR40##                                                                     ##STR41##                                                                     ##STR42##                                                                     ##STR43##                                                                     ##STR44##                                                                     ##STR45##                                                                     ##STR46##                                                                     ##STR47##                                                                     ##STR48##                                                                     ##STR49##                                                                     ##STR50##                                                                     ##STR51##                                                                     ##STR52##                                                                     ##STR53##                                                                     ##STR54##                                                                     ##STR55##                                                                     ##STR56##                                                                    ______________________________________                                    

The examples below are intended to illustrate the invention withoutrepresenting a limitation. Above and below, parts and percentages are byweight. All temperatures are given in degrees Celsius, unless indicatedotherwise. m.p. denotes melting point, c.p.=clearing point. Furthermore,C=crystalline state; N=nematic phase; S=smectic phase; and I=isotropicphase. The numbers between these symbols represent the transitiontemperatures. An denotes the optical anisotropy (589 nm, 20° C.), andthe viscosity (mm² /sec) was determined at 20° C.

"Conventional work-up" means that water is added if necessary, themixture is extracted with dichloromethane, diethyl ether, methyltert-butyl ether or toluene, the organic phase is separated off, driedand evaporated, and the product is purified by distillation underreduced pressure or by crystallization and/or chromatography. Thefollowing abbreviations are used:

    ______________________________________                                        n-BuLi    1.6 molar solution of n-butyllithium in n-hexane                    DMAP      dimethylaminopyridine                                               THF       tetrahydrofuran                                                     DCC       dicyclohexylcarbodiimide                                            ______________________________________                                    

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

The entire disclosures of all applications, patents and publications,cited above and below, and of corresponding application(s) GermanDE19541181.1, are hereby incorporated by reference.

EXAMPLE 1 ##STR57##

Step 1.1 ##STR58##

1.32 mol of n-butyllithium (15% in n-hexane) are added dropwise over thecourse of 1 hour at -70° C. under a nitrogen atmosphere to 1.2 mol of4-(trans-4-pentylcyclohexyl)-2,6-difluorobenzene in 3 l of THF. Themixture is stirred for a further hour, crushed dry ice is added at -70°C., and the mixture is stirred for a further 1.5 hours. 3 l of water areadded, the organic phase is separated off, and the aqueous phase isextracted with toluene. The combined organic extracts are subsequentlysubjected to conventional work-up.

Step 1.2

0.13 mol of 4-(trans-4-pentylcyclohexyl)-2,6-difluorobenzoic acid, 0.13mol of 2,3,4-trifluoro-phenol, 0.13 mol of DMAP and 0.15 mol of DCC arestirred at room temperature for 2 hours. The solid constituents areseparated off, and the mixture is subjected to conventional work-up. C73 N (62.6) I; Δn=+0.090; Δε=+20.9.

The following compounds of the formula I ##STR59## are preparedanalogously:

    ______________________________________                                        Example R.sup.1  L.sup.1                                                      ______________________________________                                        (2)     CH.sub.3 H                                                            (3)     CH.sub.3 F                                                            (4)     C.sub.2 H.sub.5                                                                        H      C79 I; Δn = 0.105; Δε = 23.5      (5)     C.sub.2 H.sub.5                                                                        F      C76 I; Δn = 0.078; Δε = 23.9      (6)     n-C.sub.3 H.sub.7                                                                      H      C78 N (70.9) I; Δn = 0.114;                                             Δε = 24.2                               (7)     n-C.sub.3 H.sub.7                                                                      F      C73 I; Δn = 0.090; Δε =                                   23.02                                                 (8)     n-C.sub.4 H.sub.9                                                                      F      C79 I; Δn = 0.084; Δε = 21.3      (9)     n-C.sub.5 H.sub.11                                                                     H      C74 N (84.7) I; Δn = 0.113;                                             Δε = 22.1                               ______________________________________                                    

Mixture examples

In each case, 10% by weight of a novel compound of the formula ##STR60##are added to a base mixture (BM) consisting of

    ______________________________________                                               PCH-5F   10.0%                                                                PCH-6F   8.0%                                                                 PCH-7F   6.0%                                                                 CCP-2OCF.sub.3                                                                         8.0%                                                                 CCP-3OCF.sub.3                                                                         12.0%                                                                CCP-4OCF.sub.3                                                                         9.0%                                                                 CCP-5OCF.sub.3                                                                         9.0%                                                                 BCH-3F.F 12.0%                                                                BCH-5F.F 10.0%                                                                ECCP-3OCF.sub.3                                                                        5.0%                                                                 ECCP-5OCF.sub.3                                                                        5.0%                                                                 CBC-33F  2.0%                                                                 CBC-53F  2.0%                                                                 CBC-55F  2.0%                                                          ______________________________________                                    

The physical properties of the resultant media are shown in the tablebelow.

    ______________________________________                                                  Clearing                Viscosity at 20° C.                  Mixture   point    Δn                                                                              Δε                                                                     (mm.sup.2 s.sup.-1)                         ______________________________________                                        BM        91       0.094   5.2    15                                          BM + CUZU-2-F                                                                           83.4     0.0954  8.18   15                                          BM + CUZU-3-F                                                                           86.5     0.0966  7.90   15                                          BM + CUZU-4-F                                                                           86.5     0.0960  7.94   15                                          BM + CUZU-5-F                                                                           87.6     0.0966  7.84   15                                          BM + CGZU-2-F                                                                           87.2     0.0981  8.17   15                                          BM + CGZU-3-F                                                                           90.0     0.0990  7.80   15                                          BM + CGZU-5-F                                                                           91.0     0.0988  8.01   15                                          ______________________________________                                    

Comparative example

10% by weight of the compound of the formula ##STR61## disclosed in WO12/05 230 are added to the base mixture.

The resultant mixture has the following properties:

Clearing point: 91° C.

Δn: 0.0990

Δε: 7.4

The novel compounds result in significantly greater dielectricanisotropy.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions,

What is claimed is:
 1. 3,4,5-Trifluorophenyl 4-cyclohexylbenzoates of formula I ##STR62## in which R¹ is an alkyl or alkenyl radical having up to 18 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or substituted by one or more halogen, it also being possible for one or more CH₂ groups in these radicals to be replaced by --O--, --S--, --C.tbd.C--, --OC--O-- or --O--CO-- in such a way that O atoms are not linked directly to one another, andL¹ is H or F.
 2. 3,4,5-Trifluorophenyl 4-cyclohexylbenzoates according to claim 1, in which R¹ is straight-chain alkyl having 1 to 10 carbon atoms.
 3. Liquid-crystalline medium comprising at least two mesogenic compounds, characterized in that it comprises at least one 3,4,5-trifluorophenyl 4-cyclohexylbenzoate according to claim
 1. 4. Electro-optical display containing a liquid-crystalline medium according to claim
 3. 